Ditch type floating ring for chemical mechanical polishing

Abstract

An apparatus for polishing a semiconductor wafer comprising a rotatable polishing platen having an upper surface, a polishing pad fixedly attached to the upper surface, a polishing slurry containing a mechanical abrasive deposited on the upper surface of the polishing pad. Mounted above the polishing pad is a rotatable polishing head assembly having a shallow recessed face adapted to centrally hold the upper back surface of the substrate, the recessed face is oriented substantially parallel to the upper surface of the polishing platen. The rotatable polishing head assembly has its rotatable axis offset relative to the rotatable axis of the polishing platen. A non-rotary cylindrical actuator assembly is coaxially oriented about the outer edge of the rotatable polishing head assembly with a ditched ring removably attached to the bottom surface of the cylindrical actuator assembly. The ditched ring also has a bottom section of a reduced wall thickness of approximately 5 mm. A multiplicity of conduit grooves are formed in the bottom section of the ditched ring that allows a boundary layer of abrasive slurry to travel unimpeded to the rotating semiconductor wafer. The reduced wall thickness at the bottom of the ditched ring is configured to displace wrinkles from the outer edge of the wafer to the outer periphery of the ditched ring. This solves the edge exclusion problem, while the concentric conduit grooves allow unimpeded tracks of abrasive slurry to uniformly remove microscratches from the planarized surface of the wafer

What is claimed is:  
     
       1. A method for polishing a semiconductor wafer comprising the steps of: 
       providing a chemical mechanical polishing apparatus having a rotary polishing platen with a polishing pad fixedly attached to its upper surface, and  
       a polishing slurry containing a mechanical abrasive dispensed on said upper surface of said polishing pad, and  
       a rotary polishing head assembly having a shallow recessed face adapted to centrally hold the upper back edge of said semiconductor wafer, said recessed face is oriented substantially parallel to said upper surface of said polishing platen;  
       said rotatable polishing head assembly positionally offset relative to said rotating polishing platen, and  
       providing a non-rotary cylindrical actuator assembly having a bottom surface that is coaxially oriented about an outer edge of said polishing head assembly;  
       removably attaching a ditched ring to the bottom surface of said cylindrical actuator assembly.  
     
     
       2. The method of  claim 1 , wherein said cylindrical actuator assembly is vertically floatable with respect to said polishing head assembly. 
     
     
       3. The method of  claim 1 , wherein said ditched ring further comprises: 
       a bottom section of a reduced wall thickness of approximately 5 mm;  
       a multiplicity of conduit grooves formed in said bottom section of ditched ring permitting a boundary layer of abrasive slurry to travel unimpeded to a rotating semiconductor wafer;  
       said conduit grooves formed in pairs, each groove formed on either side of a center coordinate axis of said ditched ring;  
       said conduit grooves pairs are radially concentric and developed from a point outside of said ditched ring on said center axis;  
       said center coordinate axis of said conduit grooves is coincident with rotatable axis of the polishing platen.  
     
     
       4. The method of  claim 3  wherein said conduit grooves are substantially 0.4 mm wide. 
     
     
       5. The method of  claim 3  wherein said conduit grooves are radially concentric with a spacing between of approximately 20 mm. 
     
     
       6. The method of  claim 1  wherein said reduced wall thickness at the bottom of said ditched ring is configured to displace wrinkles from the outer edge of said semiconductor wafer to the outer periphery of the ditched ring. 
     
     
       7. The method of  claim 1  wherein the use of said ditched ring during chemical mechanical polishing of wafers uniformly removes microscratches.